Various commercial technologies practiced in the energy production industry presently provide products or by-products containing carbon dioxide and lower molecular weight hydrocarbons. It has become desirable to recover these products and by-products of energy producing activities. Exemplary of such activities or processes are oil shale retorting, coal gasification, oxygen fireflooding, carbon dioxide miscible flood enhanced oil recovery other energy production techniques. Such techniques all provide a carbonaceous off-gas containing carbon dioxide, sulfur species including hydrogen sulfide and carbonyl sulfide, and hydrocarbons such as methane, ethane, and higher hydrocarbons, typically referred to as natural gas liquids, such as propane and heavier alkanes of the C.sub.4 -C.sub.7 carbon number.
Separation of such carbonaceous off-gases of energy production processes has been performed in the prior art by several techniques including refrigerated distillation, extractive distillation, amine scrubbing and semi-permeable membrane separation. These techniques have various degrees of success in producing substantially pure co-products of carbon dioxide and sulfur compounds, fuel gas compounds including methane and ethane, and heavier hydrocarbons including propane and higher alkanes. Typically, it is possible to provide products separated from such carbonaceous off-gases having substantial levels of impurity or a single high purity product wherein the penalty resides in either having substantially impure co-products of the carbonaceous off-gas treatment technique or high energy input requirements to the separation.
For instance, in the technique of carbon dioxide flooding of petroleum reservoirs to enhance oil recovery, a gas containing largely carbon dioxide with lesser amounts of C.sub.1 -C.sub.7 paraffins, water and hydrogen sulfide is produced along with the recovered oil. This gas is typically reinjected into the reservoir along with fresh makeup carbon dioxide from an external source to maintain reservoir productivity. Dehydration, sweetening and hydrocarbon removal are required to some extent for purposes of corrosion control, safe pipelining and proper miscibility of the gas with the petroleum underground. The recovery of larger amounts of valuable hydrocarbons as a by-product, however, is very attractive economically. Furthermore, production of a gaseous light hydrocarbon product of low hydrogen sulfide and carbon dioxide content is desirable, since this stream may then be burned as a fuel without environmental problems or sold via pipeline to a natural gas network. Also, high carbon dioxide recovery for reinjection is important to overall economics of the enhanced oil recovery project.
The various processes briefly outlined above have been available to the energy production industry to effect the separation of hydrocarbons from the bulk carbon dioxide in the produced gas of an enhanced oil recovery process or other energy production techniques. However, these processes are either energy or capital intensive (or both) or are unable to economically achieve a high degree of separation.
For example, in refrigerative distillation, the produced gas is partially condensed by external refrigeration to provide a liquid reflux of primarily carbon dioxide to wash propane and heavier hydrocarbons down the column. Carbon dioxide product leaves the distillation column overhead along with essentially all of the hydrogen sulfide, methane and ethane. Recovery of the propane is dependent upon the carbon dioxide loss acceptable in the hydrocarbon product leaving the bottom of the distillation column and ultimately with the fuel gas. Typically, the propane recovery is low (on the order of 20-30%). For higher hydrocarbon recovery, the refrigeration energy requirements increase significantly, such that more incremental energy is put into the process then is recovered as additional hydrocarbon product. A natural gas liquid product is also produced, consisting mainly of butane and heavier hydrocarbons.
Alternatively, the prior art has utilized extractive distillation that uses an additive, generally a C.sub.3 -C.sub.6 alkane mixture, to effect a separation of carbon dioxide, light gas (methane and ethane) and natural gas liquids (propane and heavier hydrocarbons). High recovery of carbon dioxide, methane/ethane and C.sub.3 is possible, but at the expense of high energy consumption. However, the extractive distillation processes known in the prior art are also capital intensive.
Various ethanolamine scrubbing techniques are also utilized to perform such separations wherein an amine solvent in an absorption/regeneration cycle is used to extract carbon dioxide, the bulk component, from produced gas. The processes are recited to reduce energy costs since the amine solvent regeneration is accomplished or partially accomplished by pressure reduction, rather than steam stripping as in diethanolamine systems. However, a significant energy and capital expenditure is required for enhanced oil recovery applications to compress the resulting carbon dioxide product, which is produced at low pressures before it can be utilized for reinjection at high pressure into an oil producing geologic formation. The carbon dioxide product will also contain all of the hydrogen sulfide in the feed gas. The hydrocarbon enriched gas leaving the amine absorber at high pressure must be further processed to recover any desired liquid hydrocarbon products. The steam stripped diethanolamine systems are energy intensive because of the required steam for stripping or regeneration requirements.
A typical refrigerated distillative separation of carbonaceous off-gas is set forth in U.S. Pat. No. 4,417,449 wherein carbon dioxide, a light fuel gas and a heavy hydrocarbon stream are separated in an autorefrigerated distillative separation. However, this technique results in a fuel gas having a higher than economically desirable carbon dioxide content, which lowers the BTU value of the fuel gas and a carbon dioxide stream which does not result in high recoveries of the total carbon dioxide content being processed by the distillative separation apparatus.
Various membrane techniques are presently described in the literature for separating carbon dioxide and hydrocarbons by differential rates of permeation of carbon dioxide through the membrane relative to other gas constituents. The carbon dioxide is recovered at low pressure and must be recompressed for reinjection into a carbon dioxide utilizing process, such as the enhanced oil recovery operations utilizing carbon dioxide miscible flood. Both the compressor equipment and the membrane are high capital cost items and staging of the membranes is frequently required for high carbon dioxide recovery at high purity. Typical membranes are disclosed in the following articles:
Membrane Separation Processes For Acid Gases by S. S. Kulkarni, E. W. Funk and N. N. Li, Corporate Research UOP, Inc., Des Plains, Ill. and R L. Riley of Fluid Systems Division UOP, Inc., San Diego, Calif., given at the AICHE Summer National Meeting, Denver, Aug. 28-31, 1983; Separation Techniques, Membranes For Natural Gas Sweetening and Carbon Dioxide Enrichment by William H. Mazur and Martin C. Chan, Envirogenics Systems Company, El Monte, Cailf., reprinted from Chemical Engineering Progress, October 1982; Membrane and Separation Technology News, Vol. 2, No. 10, June 1984; Spiral Wound Membranes, Carbon Dioxide Removal Process for the Well Head, presented by Thomas E. Cooley to AICHE 1984 Spring National Meeting, May 21, 1984, Anaheim, Calif.; Membrane Separation of Carbon Dioxide and Hydrogen Sulfide from Natural Gas--Field Experience, by Nicholas R. Grey and William H. Mazur, presented at the American Institute of Chemical Engineers, 1984 Spring National Meeting, Anaheim, Calif., May 20-23, 1984; Applications of Prism.TM. Separators in Natural Gas Service, by D. J. Stookey, T. E. Graham and A. P. Aneja, American Institute of Chemical Engineers, 1984 Spring National Meeting, Anaheim, Calif., May 20-23, 1984 and SEPAREX System Makes Hydrocarbon Recovery Feasible, by Brian D. Miller, Rudolph Richards, Mark E. Schott, 1984 Spring National Meeting American Institute of Chemical Engineers, Anaheim, Calif., May 20-23, 1984. All of these membrane separation disclosures recite the potential separation of carbon dioxide from hydrocarbons using one or more staged semi-permeable membrane filters which are selective to carbon dioxide permeation, in some instances in conjunction with drying and amine or glycol contact.
However, all of the above prior art techniques either fail to provide high purity fuel and carbon dioxide products at high recoveries or require undesirable levels of capital investment or energy utilization to effect such result. The present invention disclosed herein separates and recovers the majority of C.sub.1 -C.sub.7 hydrocarbons from the bulk carbon dioxide, recovers a high percentage of the carbon dioxide for reinjection or reutilization and achieves acceptable product specifications in a manner which requires less energy and capital than competing processes. This unique combination of separatory techniques using low temperature temperature distillation and semi-permeable membrane separation of initial products taking advantage of differential product stream pressures provides desirable product purity and high recovery of components of carbonaceous off-gas, such as is typically required for carbon dioxide miscible flood enhanced oil recovery operations.